Accurate regulation of cell death is critical for the development and adult function of multicellular organisms. For many, if not all cells, survival is regulated by a balance of death inhibitory and death activation signals present in the cytoplasm. The relative levels of these activities, and thus whether a cell chooses to live or die, are determined by signals from the cell's external environment and by internal cues. Major questions are to understand how these disparate signals, presumably mediated by distinct signal transduction cascades, can regulate a common death effector machinery. Work from many organisms show that cell death is under genetic control, and that regulators of cell survival and death are evolutionarily conserved. These observations support the hypothesis that a universal cell death program exists in multicellular organisms. The Inhibitor of Apoptosis (IAP) family of proteins, originally identified as baculovirus encoded inhibitors of a host suicide response, are an important, evolutionarily conserved family of cell death inhibitors. Drosophila IAPs, DIAP1 and DIAP2, were identified during genetic screens for enhancers of a small eye phenotype associated with a hyperactivated cell death signaling pathway and by data base searchers, respectively. DIAP1 is essential for the survival of many cells in the fly, and expression of viral, Drosophila and mammalian IAPs is able to block cell death in response to diverse stimuli. Also, mutations in the human and mammalian IAPs is able to block cell death in response to diverse stimuli. Also, mutations in the human Neural Apoptosis Inhibitory Protein gene, an IP homologous gene are associated with spinal muscular atrophy, a common human neurological degenerative disease. These IAPs are an important family of cell death regulators. This proposal describes experiments designed to identify contexts in which DIAP death preventing activity is important, mechanism by which DIAPs function to block cell death, and ways in which their function is regulated. This proposal also describes the design and implementation of an over- expression-based genetic screen that allows us to individually drive the eye-specific expression of large numbers of genes in different genetic backgrounds, utilizing a transposable element that contains an eye- specific promoter at one end. By mobilizing this element throughout the genome we can quickly screen a large fraction of the genome of over- expression-dependent regulators of cell death in various contexts. Because the P element tags the site of insertion we can quickly identify the affected gene.